Conventional strollers for carrying a baby usually have a linkage bar consisting of rods pivotally engaging with one another. A release mechanism is used to control a joint assembly located at the juncture of the rods. The joint assembly has a latch mechanism which may be released or latched to collapse the stroller to a folding position or unfold the stroller to an extended position. For instance U.S. Pat. No. 5,772,235 discloses a convertible stroller. However, it has a complicated frame structure consisting of coupling linkage bars. This is quite difficult to assemble. As it has many rod elements, the total weight of the frame structure is greater. This is not easy to carry and move. Folding movement cannot be smoothly executed and could result in a troublesome operation.